The question remains as yet unresolved as to the exact nature of the relationship between metabolic control and microvascular complications in type I diabetes. This proposal is to participate in the Diabetes Control and Complications Trial--Phase III. During this phase, 30 subjects with IDDM will be randomized into conventional or intensive treatment regimens. Over a minimum of five years these subjects will be evaluated for metabolic control and microvascular cmplications (retinopathy, neuropathy and nephropathy). The standard treatment group (control) will be followed at three month intervals with the aim of therapy to maintain the subjects free of symptoms of hypo- and hyperglycemia. Treatment for this group is characterized as one to two injections of mixed insulin with capillary blood testing. The intensively treated group (experimental) will be followed at least monthly with the aim of achieving and maintaining euglycemia for the entire study period. For those intensively treated, multiple injections of insulin or continuous subcutaneous insulin infusion combined with frequent capillary blood testing will be employed. Microvascular complications will be assessed at entry, periodically throughout the study and at the five year point. Treatment will be instituted and the subjects will be followed on the Model Demonstration Unit of the Diabetes Research and Training Center. A multidisciplinary team comprised of pediatric and adult diabetologists, nurse clinical specialists, nutritionists and psychologists constitute the primary health providers. Our goal will be to sustain near normal levels of glycemia in the experimental group in order to clarify the relationship between control and complications.